1. Filed of the Invention
The present invention relates to an analog/digital (A/D) converter apparatus and, more particularly, to an analog/digital converter apparatus having a high-precision A/D conversion function which is performed in consideration of an influence of device mismatch caused by manufacturing a semiconductor integrated circuit.
2. Discussion of the Background
The advance of manufacturing technology of semiconductor integrated circuits has enabled a high-speed processor to be manufactured inexpensively. Such a processor has recently been incorporated into household electrical appliances and communications equipment to process a number of signals in digital form. It is thought that digital signal processing will be advanced from now on and its share will be increased even in the field of communications system. To achieve sophisticated signal processing, a processing apparatus has to be operated with high precision as well as at high speed. In systems of such a field, an analog signal is used to receive a signal or to interface with the outside. A low-cost, high-speed and high-precision analog/digital converter apparatus is therefore required in order to spread digital signal processing more and more in the future.
A representative example of an analog/digital converter apparatus is a flash type analog/digital converter apparatus for comparing the level of an input analog signal with that of a reference signal and converting it into a digital signal. This type of converter apparatus executes simple processing of comparing an input signal with plural reference signals at once and is therefore suited to an application area requiring high-speed processing. Factors for determining conversion precision of the flash type analog/digital converter apparatus are equivalent input offset of a comparator section for determining a relationship in voltage level between an input analog signal and a reference signal and resolution of a reference voltage generator. When an analog/digital converter apparatus is realized in a semiconductor integrated circuit, both the factors of equivalent input offset and resolution are due to device mismatch caused by manufacturing the semiconductor integrated circuit. Since, at this point in time, the equivalent input offset is more remarkable as a factor for determining the above conversion precision than mismatch of elements constituting the reference voltage generator, the conversion precision depends upon the equivalent input offset.
A prior art analog/digital converter apparatus adopts a method of relaxing an influence of equivalent input offset of a comparator. In this apparatus, for example, an analog input voltage and a reference voltage are not compared directly, but an output of a differential amplifier receiving both a reference voltage and an analog input voltage is added to outputs of a plurality of differential amplifiers adjacent thereto and a result of this addition is compared with the above output of the differential amplifier (disclosed in Kaneko et al., "A/D CONVERTER," Jpn. Pat. Appln. KOKAI Publication No. 6-350452). The equivalent input offset can thus be averaged by the number of added outputs of the differential amplifiers.
In the above method, however, the number of outputs used for averaging the equivalent input offset is two or three and thus an influence of device mismatch still remains.
Furthermore, an adder circuit with considerable precision is needed to add output signals of the plural differential amplifiers, as is time required for processing them. In other words, it is understood that a great improvement in conversion precision cannot be expected though additional processing is given to a system for processing a plurality of signals at once.
Even though a problem of equivalent input offset of comparators is resolved, precision is still restricted by mismatch of elements constituting a reference voltage generator.
Therefore, unless the problems of both equivalent input offset and device mismatch are overcome fundamentally, a high-precision analog/digital converter apparatus is very difficult to achieve.
If, as described above, an analog/digital converter apparatus is constituted of a semiconductor integrated circuit, it causes a problem of device mismatch and it is difficult to achieve at low cost and with high conversion precision.